The present invention relates to the control of an optical amplifier, and more particularly, to ways in which to monitor and control an optical amplifier for use in fiber-optic communications systems.
Optical amplifiers are used to amplify optical signals. A typical optical amplifier may be based on fiber that has been doped with rare-earth elements such as erbium. Fiber amplifiers are typically pumped by diode lasers.
In some cases, it is desirable to control an optical amplifier in real-time based on an analog input signal to achieve a particular amplification characteristic. However, this is difficult to do, particularly when the analog input signal varies over a large dynamic range. Consider for example a fiber-optic communication system that transmits wavelength division multiplexed (WDM) signals, in which multiple wavelengths of light are used to support multiple communications channels on a single fiber. An optical amplifier used to amplify such a WDM signal should be controlled to maintain constant per channel gain even if the power of the WDM signal changes, for example, due to a fiber cut, an equipment failure, or channel add/drop.
WDM systems are expected to support as many as 40 WDM channels, 160 channel systems in the future. Each WDM channel carries an optical signal that may vary in power from zero to some maximum value. For example, in some situations, a weak signal carried in an optical channel may have one-sixth (⅙) the power of a strong signal. Consequently, the power of a WDM signal may vary over a large range of powers. It is difficult to control an optical amplifier to maintain a constant gain per channel when the input power varies over such a large range.
It is an object of the present invention to provide low-error, real-time control of an optical amplifier in response to an analog input signal having a large dynamic range.
These and other objects of the invention are accomplished in accordance with the principles of the present invention by providing an optical amplifier with an amplifier control that is capable of responding to an analog input signal having a large dynamic range with low relative quantization error. According to one exemplary embodiment, the amplifier control includes a non-linear scale analog-to-digital converter that produces a digital signal in which the increment between successive bits tracks a non-linear scale rather than a linear scale. For example, the non-linear scale analog-to-digital converter may be a logarithmic scale analog-to-digital converter, in which the increment between successive bits tracks a logarithmic scale.
According to a further exemplary embodiment, multiple analog-to-digital converters are used to produce digital signals corresponding to different scales. A controller selects the output of one of the analog-to-digital converters to generate control signals for controlling the optical amplifier.
The amplifier control of the present invention may be responsive to any analog input signal. For example, the amplifier control may be responsive to the optical input signals supplied to the optical amplifier, the amplified output signals produced by the optical amplifier, or both. Alternatively, or in addition, the amplifier control may be responsive to optical signals elsewhere in an optical system.
The amplifier control of the present invention may be used to control various types of optical amplifiers, such as rare earth-doped fiber amplifier, Raman amplifiers, parametric amplifiers, and/or semiconductor optical amplifiers, to name a few. The amplifier control may control a variety of optical amplifier parameters. For example, the amplifier control may control the pump power applied to an erbium-doped fiber amplifier (EDFA). Alternatively, the amplifier control may control the bias voltage of a semiconductor optical amplifier.
The optical amplifier may be used in various optical systems. One suitable system is a WDM fiber-optic transmission system. For example, the optical amplifier with optical control may be used to amplify WDM optical signals at points between network nodes. The optical amplifier control can control the optical amplifier to approximate a constant optical gain per channel for a large number of channels, even if channels are added or dropped.
Further features of the invention and its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.